1. Field of the invention
The present invention relates to a storage medium for storing informations, and more particularly to an optical storage medium for storing and reproducing informations upon irradiation of a laser beam.
2. Description of the Related Art
An optical disk comprises a substrate, an optical recording layer overlying the substrate, and an light transmitting layer overlying the optical recording layer.
It is desirable to improve a recording density of the optical disk which includes lands and grooves. Japanese laid-pen patent publication No. 4-079483 discloses that both the lands and grooves are used for recording informations to increase the recording density by approximately two times of when only either the lands or the grooves used for recording informations.
It is also know as another conventional measure for improving the recording density to increase a numerical aperture of an objective lens performing as a condensing lens. The increase in the numerical aperture of the objective lens decreases a beam spot area on the optical disk, thereby improving the recording density.
For reproducing the information from the compact disk (CD), the numerical aperture is 0.45. For reproducing the information from the digital versatile disk (DVD), the numerical aperture is 0.6. In recent years, an optical disk with a high numerical aperture of 0.85 has been proposed.
The high numerical aperture for realizing the highly dense recording may decrease an acceptable range of an aberration which is caused by the fact that the normal of a surface of the disk is tilted from an optical axis of the condensed laser beam. For this reasons, it is preferable for the high numerical aperture that the substrate of the optical disk is thin. A substrate thickness for CD is 1.2 millimeters, whilst a substrate thickness for DVD is only 0.6 millimeters.
If the numerical aperture is increased up to at least 0.85, then after the recording layer has formed over the substrate, an light transmitting layer with a thickness of about 0.1 millimeter is then formed over the recording layer, so that a laser beam is irradiated through the light transmitting layer onto the recording layer for recording and reproducing the informations.
The combined use of the lands and the grooves for recording and reproducing the informations and the increase in the numerical aperture of the optical disk allow that the recording density is increased by about 4 to 5 times. This structure, however, causes the following disadvantages. FIG. 1 is a fragmentary cross sectional elevation view illustrative of a conventional structure of an optical storage medium allowing lands/grooves recordings. An medium 100 comprises a substrate 101, a reflective layer 102 over the substrate 101, a first dielectric layer 103 over the reflective layer 102, a recording layer 104 over the first dielectric layer 103, a second dielectric layer 105 over the recording layer 104, and an light transmitting layer 106 over the second dielectric layer 105.
The substrate 101 has substrate grooves 101a with a substrate groove width 110a and substrate lands 101b with a substrate land width 110b. The substrate groove width 110a is defined to be a distance between two intermediate points of adjacent substrate sloped surfaces defining the substrate groove 101a. The substrate land width 110b is defined to be another distance between two intermediate points of another adjacent substrate sloped surfaces defining the substrate land 101b. The substrate groove width 110a is equal to the substrate land width 110b. 
The medium 100 has an effective groove width 111a and an effective land width 111b. The effective groove width 111a is defined to be a distance between two intermediate points of adjacent sloped surfaces of the recording layer 104. The effective land width 111b is defined to be another distance between two intermediate points of adjacent another sloped surfaces of the recording layer 104. Therefore, the effective groove width 111a is different from the substrate groove width 110a. The effective land width 111b is different from the substrate land width 110b. The effective groove width 111a is narrower than the substrate groove width 110a. The effective land width 111b is wider than the substrate land width 110b. Since the substrate groove width 110a is identical with the substrate land width 110b, the effective groove width 111a is narrower than the effective land width 111b. 
A difference in width between the effective groove width 111a and the effective land width 111b is increased as a distance in vertical direction between the substrate 101 and the recording layer 104 is increased. The recording layer 104 is separated by double layers, for example, the reflective layer 102 and the first dielectric layer 103 from the substrate 101. Further, the reflective layer 102 is thick as being larger in step coverage than the first dielectric layer 103. Those results in that the distance in vertical direction between the substrate 101 and the recording layer 104 is relatively large.
Since the effective groove width 111a is narrower than the effective land width 111b, the following disadvantages are raised. Recording marks are formed on both grooves and lands of the recording layer 104. A signal amplification from the recording mark on the narrow groove of the recording layer 104 is smaller than another signal amplification from the other recording mark on the wide land of the recording layer 104. A difference in signal amplification between the recording marks on the narrow groove and the wide land of the recording layer 104 means it difficult for the conventional medium to obtain a desirable uniform quality of both signals from the groove recording mark and the land recording mark.
In the above circumstances, the development of a novel optical storage medium free from the above problems is desirable.
Accordingly, it is an object of the present invention to provide a novel optical storage medium free from the above problems.
It is a further object of the present invention to provide a novel optical storage medium for recording and reproducing informations onto and from both grooves and lands at a uniform signal quality.
It is a still further object of the present invention to provide a novel optical storage medium for recording and reproducing informations onto and from both grooves and lands, wherein the medium is suitable for a highly dense recording.
The present invention provides an optical storage medium including: a substrate having grooves and lands; a first dielectric layer overlying the substrate; a recording layer overlying the first dielectric layer; a second dielectric layer overlying the recording layer; and a light transmitting layer overlying the second dielectric layer, wherein each of the grooves is larger in width than adjacent two of the lands.
The above and other objects, features and advantages of the present invention will be apparent from the following descriptions.